Mangroves 2

7/27/2019 Mangroves 2

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History

Mumbai Historical records indicate that there were several islands around Mumbai during 1670.However, the Britishers, who were ruling the country identified the importance of these islandsfor commercial purpose. They deforested the fringing mangroves and reclaimed these islandsinto one continuous landmass, which later came to be known as "Greater Bombay". Since thenthe developmental and bsequently population pressure rapidly increased and being the coastalarea, it took the toll of mangrove land. During the process of deforestation and reclamation, afew mangrove patches are still left in the heart of the city, which proves that today's megacityhad a luxuriant past of mangrove forests (fig). Major mangroves are seen today in Mumbaialong the Vasai Creek, Thane Creek, Manori and Malad, Mahim - Bandra, Versova, Siwari,Mumbra - Diva and few more places.

Importance of Mangroves for Mumbai

Mangroves represent the spirit of Mumbaithey are plucky survivors. But each day,millions of citizens in Mumbai pass thesehardy plants imagining they are little morethan dirty, muddy weeds growingpointlessly along the shoreline. How littlepeople understand just how importantmangroves are to the quality of life of thecitizens of Mumbai.

By trapping silt, mangroves maintain theintegrity of Mumbais shoreline. This is avital service to the city of Mumbai as it isvery prone to erosion, having been builton reclaimed land that is battered by the

sea on all three sides. The recent rains in Mumbai and the disaster that followed demonstratedthe consequence of tampering with the ecology of fragile ecosystems like mangroves. HadMumbais Mithi river and Mahim creek mangroves not been destroyed by builders, fewer peoplewould have died and the property damage would have been dramatically less.

The Koli community in Mumbai worships mangroves because they know that these are breeding

and nursery grounds for the marine organisms on which their sustenance depends.

Mangrove community of Mumbai

In the early nineties, perhaps over 37 sq. km. of mangroves existed in Mumbai, largely in theThane creek, Mahim, Versova, Gorai and Ghodbunder, with sporadic patches in places such asBandra, Malabar Hill and Colaba. Mumbai has probably lost 40 per cent of all its mangroves in


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the past decade or so, largely because of reclamation for housing, slums, sewage treatmentand garbage dumps. Fortunately, thanks to the Godrej family, we still have excellent mangroveforests in Vikhroli (Link).

Around 20 out of the 35 species of true mangroves found in India have been identified along theMaharashtra coast and 15 species of these are found in Mumbai.

Because of the high salinity of the soil, something like 60 per cent of Mumbai mangrovescomprise Avicennia marina. Nor surprisingly this species also tolerates pollution including heavymetals such as lead, mercury and chromium, all found in significant concentrations in the Mithiriver.


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Mangrove Ecology...........

Where do Mangroves occur

The richest mangrove communities occur in tropical and sub-tropical areas, i.e., between the30N and 30S latitudes where the water temperature is greater than 24C in the warmestmonth, where the annual rainfall exceeds 1250mm and mountain ranges greater than 700mhigh are found close to the coast. Mangroves are found practically in almost all the continents,excepting Europe, the Arctic and Antarctic. Luxuriant patches of mangroves are found on all theother continents but the best mangroves are found in Asia, especially in India and Bangladesh -the Sunderbans are the largest mangrove forest in the world both in size as well as biodiversity

The total area of mangroves in India is about 6,740 sq. km, which is about 7% of the world'stotal area of mangroves. Of the total mangroves 80% are present along the east coast, mostlyforming the Sunderbans, Bhitarkanika and the Andaman & Nicobar mangroves. The GangeticSunderbans is about 4,000 sq. km whereas Andaman & Nicobar is about 700 sq km. Besides,large rivers like Mahanadi, Krishna, Cauveri, Godavari also harbour major mangroves in theirestuarine regions.

The remaining 20% mangroves are scattered on the west coast from Kutch to Kerala. Thereason for such a restricted mangrove cover is the peculiar coastal structure and the nature ofestuaries formed by the relatively small and non-perennial rivers except Narmada and Tapi.

How do they establish?

Under the right conditions like the formation of a mud-flat, growth of mangroves is initiated.Stabilization of mud-flats is a preliminary process in the establishment of mangroves. Pioneerplant species initiate this process. The roots of these plants help in binding the soil and alsohelp the establishment of micro-organisms which further help in stabilizing the area.Stabilization starts from the land side and gradually shifts towards the sea. The pioneer plantsare species like Porterasia coarctata and some members of the Cyprus family. These are slowlyreplaced by other mangrove plants and then these mangroves gradually spread towards thesea.

Once mangroves grow, the submerged banks are fully stabilized. Then the plants slowly reach astage which is called the climax vegetation. A climax vegetation of mangroves is represented bythe complete circle of life where there are different species of plants, animals (both terrestrialand aquatic) and micro-organisms forming an ecosystem called the tropical salt marsh or themangrove ecosystem. In case the sediments are not stabilized, submerged banks are washedout. Thousands of deltas are formed and washed out every year before they can be stabilized.In the Gangetic delta this situation is quite common.

Zonation in Mangroves

Mangal along a tropical bay characteristically shows zonation. In India this zonation may be very


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distinctive (east coast of India) or merging (west coast of India). A very broad and generaldistinction would be:-

1. Proximal Zone (Front mangroves)This zone is towards water front, subject to regular tidal effect where intensity of soil

accumulation and inundation is a continuous process. The mangrove species in this zone arespecially adapted with stilt roots, prop roots for stability and anchorage. Main species with thesefeatures are Rhizophora apiculata and Rhizophora mucronata. On rocky and coral reefsubstrata, Avicennia Spp, Sonneratia Caseolaris are also found. Both Avicennia and Sonneratiaproduce pneumatophores.

2. Middle Zones (Mid mangroves)Above the Rhizophora/ Avicennia line luxuriant group of Bruguiera gymnorrhiza, B. Cylindrica,Lumnitzera racemosa, L. littoralis, Ceriops tagal and Aegiceras corniculatum occur. Ceriops andBruguiera develop a strong hold fast in the form of knee roots or bent roots as a specialadoption for supporting the erect bole.

3. Distal Zone (Back mangroves)Towards island area mangroves like Excoecaris agallocha, Heritiera littoralis and Xylocarnusspp occur. Both Heritiera and Xylocarpus produce buttresses. Generally the salinity is on lowerside in this zone occurring towards hill sides where run off of fresh water is for a prolongedperiod. The duration of tidal submersion is low in this zone compared to front mangroves.

However, the zonation in mangroves is not so simple and varies from place to place. Everyspecies has its own level of salinity tolerance. Estuaries on east coast show distinct zonation.The high salinity range on the east coast estuaries may be the principal reason for distinctzonation there. The range and force of tidal action also play a determinant role in creation and

maintenance of zones as distribution of seeds or propagules is influenced by tidal action. Also,tides do influence the salinity in an estuary.

Mangrove Adaptations

Mangrove plants live in hostile environmental conditions such as high salinity, hypoxic (oxygendeficient) waterlogged soil strata, tidal pressures, strong winds and sea waves. To cope up withsuch a hostile environment mangroves exhibit highly evolved morphological and physiologicaladaptations to extreme conditions.

Do mangroves need salt?

The answer is no. Mangroves are facultative halophytes, i.e., the presence of salt in theenvironment is not necessary for the growth of mangroves and they can grow very well infreshwater. One particular advantage to growing in a salty environment is the lack ofcompetition! Only a limited number of plants have invested evolutionary energy into adapting tointertidal conditions. In the optimum conditions of a tropical rainforest, diversity is great andcompetition fierce.

How do Mangroves cope with salt


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Coping with salt

The first line of defense for many mangroves is to preventmuch of the salt from entering by filtering it out at root level.Some species can exclude more than 90 percent of salt inseawaters (Rhizophora, Ceriops, Bruguiera species are allsalt-excluders.)

Another method is the retention of water in the leaves givingrise to leaf succulence in many species, viz., Sonneratia apetala, S. alba, Lumnitzera recemosa,Salvadora persica etc. These species show remarkably high concentration of salts stored intheir tissue. To avoid the toxic effects of salts, these plants absorb a large quantity of water fordilution of salt.

The leaves of many mangroves have special salt glands, which are among the most activesalt-secreting systems known. It is quite possible to see and/or taste the salt on the leafsurfaces of species, which choose this method. (Examples of salt-secretors include Avicennia,Sonneratia and Acanthus).

Fourth method of coping with salt is to concentrate it in bark or in older leaves which carry itwith them when they drop. (Lumnitzera, Avicennia, Ceriops and Sonneratia species all use this).

Specialized Root System in Mangroves

Specialized Root System

The major plant species forming the mangroveecosystem have aerial roots, commonly prop

roots or even stilt roots (Example: Rhizhophoraspp). Stilt roots serve, of course, to anchor theplants, but also are important in aeration,because the mangrove mud tends to beanaerobic.

Rhizophora spp (Red mangroves) have proproots descending from the trunk and branches,providing a stable support system. Othermangrove species, including the whitemangroves (A. marina) obtain stability with anextensive system of shallow, underground

cable roots that radiate out from the centraltrunk for a considerable distance in alldirections: pneumatophores extend from thesecable roots.


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Breathing Roots (Pneumatophores):Special vertical roots, called pneumatophores,form from lateral roots in the mud, oftenprojecting above soil (to a height of 20-30cms, e.g. Avicennia, Sonneratia ) permittingsome oxygen to reach the oxygen-starvedsubmerged roots. Roots also can exhibitdevelopment of air cavities in root tissues,designs that aid oxygenation of the tissues.The density, size and number ofpneumatophores vary per tree. They aregreen and contain chlorophyll.

Stilt rootsare the main organs for breathingespecially during the high tide. They are verycommon in many species of Rhizophora andAvicennia (Avicennia marina and Avicennia

offficinalis). The stilt roots of Rhizophoramucronata extend more than a meter above

the soil surface and contain many small pores(lenticels) which at low tide allow oxygen to

diffuse into the plant and down to theunderground roots by means of open

passages called aerenchyma. The lenticels arehighly hydrophobic and prevent water

penetration into the aerenchyma system duringthe high tide. In Brugeira and Ceriops they

become hollow and malfunctional after somestage.

Aeration occurs also through lenticels in thebark of mangrove species, e.g., species of

Rhizophora.

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Reproductive Strategies of Mangroves

Reproductive Strategies:

Virtually all mangroves share two common reproductive strategies: dispersalby means of water and vivipary.

Members of the Rhizophoraceae family (Rhizophora, Bruguiera and Ceriopsspecies) have an intriguing viviparous method for successfully reproducingthemselves. Vivipary means that the embryo develops continuously whileattached to the parent tree and during dispersal. They may grow in place,attached to the parent tree, for one to three years, reaching lengths of up toone meter, before breaking off from the parent and falling into the water.

These seedlings (propagule) then travel in an intriguing way. In buoyant sea

water they lie horizontally and move quickly. On reaching fresher (brackish)water however, they turn vertically, roots down and lead buds up, making it easier for them tolodge in the mud at a suitable, less salty. Once lodged in the mud they quickly produceadditional roots and begin to grow.

Some other species (Avicennia and Aegiceras) also produce live seedlings but these are stillcontained within the seed coat when it drops from the plant. The seed of Avicennia floats untilthis coat drops.

A brief account of fauna in Mangrove ecosystem

Zooplankton

The zooplankton in the mangrove areas mostly includes crustacean larvae. Larvae of severalspecies are found in large quantities. This is obvious because mangroves are the breedingground for a variety of organisms. Food in the form of suspended solids is plenty, while shelteris sought in the complex root-systems of plants.

InsectsInsect fauna of mangroves has not been adequately researched in India. Hardly any informationis available about the insects in mangrove areas. But some work has been conducted in W.

Bengal and Orissa where honey collection is one of the major tribalactivity. Collection of fine quality mangrove honey is a majoroccupation for tribals. The common honey bees found here are Apisdorsata (rock bee) and Apis mellifera (European bee).

Butterflies and mothsare also commonly found in the mangroveecosystem. Several species of butterflies and moths have beenreported in mangrove areas.

Salmona is a butterfly which is associated with the mangrove associate, Salvadora.


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Another moth, Hybloea puera, has recently been observed to be infesting large tracts ofAvicennia marina on the Western coast.

Molluscs and Crustaceans

Mangroves are a paradise for aquatic animals like molluscs and crustaceans. About 20 speciesof shellfish and 229 species of Crustaceans belonging to Indian mangroves have beenrecorded. However, much work is needed on this subject. The reason for high density of theseanimals lies in the high deposits of silt and detritus in the mangrove environment. In addition tothe rich sediments brought by the rivers, the leaves shed by mangrove trees also add in theorganic constituents of mangrove soil. As a result, a rich source of food is created which isutilized by detritovorous organisms like crustaceans and molluscs. Many crustaceans in themangroves make burrows which are used for refuge, the feeding, as a source of water or forestablishing a territory necessary for mating. Some may filter water through their burrows,feeding on suspended detritus and plankton while others may breed there. These burrows playan important role in the mangroves, aerating, draining and turning the dense waterlogged soil -a direct benefit to the plants which in turn give them shelter.

Mangrove Crab (Scylla cerata) :Scylla serrata, the large edible swimming crab, inhabits themuddy bottom of mangrove estuaries, as well as coastal brackish water. Due to its associationwith mangroves it is known as the Mangrove Crab or the Mud Crab. This is a commerciallyimportant crab and it is trapped in special nets throughout the country. Due to its habit of cuttingstems of young plants it is a pest for young plantations.

Fiddler Crab (Uca sp.) : Fiddler crab is probably one ofthe first animals one sees in a mangrove area. Fiddlercrabs are charcteristised by the males which are armouredwith a single huge pincer (claw) which is used as acourtship display tool rather than for protection. The other

pincer is small in the males and is used for feeding.Females have two small pincers of equal size. Fiddlercrabs are semiterrestrial and prefer to stay on protected sand and mud beaches of bays andestuaries. Their burrows are located in the intertidal zone, and at low tide the crabs come out forfeeding and courting. There are 62 known species of fiddler crabs in the world.

Fiddler crabs are preyed upon by fish, large crabs, some mammals and birds. In the Indo-Pacific region a species of snake called "mangrove snake" goesdown into a fiddler crab's burrow and hunts the crab.

Telescopium telescopium :Like the mangrove crab, thischaracteristic mollusc is also strongly associated with mangroveenvironment and is an indicator species for mangroves. The nameis derived from the typical telescope-like structure of the shell. Thisanimal belongs to Class Gastropoda of Phyllum Mollusca.


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Fish :Mangroves are the breeding and nursery grounds for several species of fish. There are atotal of 105 species of fish which are typical mangrove dwellers in India. Besides, many otherspecies visit the mangrove environment frequently or occasionally. Some common species are -

scats, milk fish, mudskippers, mullets, cat fish, perches, etc.

Mud skippersare one of the fish which live on themud flats associated with mangroves shores. The mudskipper is a fish well adapted to alternating period ofexposure to air and submersion and is frequently seenhopping along the mud at the water's edge They arewell-comouflaged and able to change colour to matchtheir background. It respires under water like other fishbut out of the water gulp air. When submerged it swims like a fish but on land proceeds by aseries of skips. Some of them can even climb trees using their fused pelvic (rear) fins assuckers and their pectoral fins as grasping 'arms'.

When a mud skippers is out of water it carries in its expanded gill chamber a reserve from whichto extract oxygen. After a few minutes, when this reserve is exhausted, it is replenished frompool or from water in the burrows which they dig. The mud skipper's most noticeable feature is apair of highly mobile eyes perched on top of the head to increase the field of view and to enableit to see both under and over the water.

Mangroves 2

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